The present invention relates to a wear-resistant hard metal body and to a method for producing such a hard metal body.
It has long been known that hard metal bodies can be formed from at least one binder or bonding metal of iron, cobalt and nickel and at least one hard metal refractory carbide of at least one of the elements titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum and tungsten. The hard metal body generally is formed by uniting a powdered form of the hard metal carbide by compression with the binding metal, followed by sintering. During the sintering process, the product generally receives its final shape and dimensions and the resulting sintered product is a molded, shaped, hard metal body which often is referred to as a cemented carbide. The hard metal bodies possess great hardness and find wide application in metal turning and cutting tools which are hard enough to permit high turning and cutting speeds in rock or metal.
Increasing demands have been placed on hard metal bodies and there has been a continuing search to provide hard metal bodies having still greater wear resistance. To this end, there has been produced hard metal bodies comprising a core of a shaped, hard metal body formed from a hard metal carbide and bonding metal as described above and a surface coating of a hard material on the core. The surface coating of hard material has been made from such materials as carbides, nitrides, borides and/or oxides. Preferably, the surface coating has been made from titanium carbide.
Moreover, the surface coating can be made from all carbides, nitrides and borides of the Group IVa to VIa of the periodic system of elements, such as hafnium carbide, tungsten carbide, zirconium nitride, hafnium nitride, niobium nitride, tantalum nitride, titanium nitride, titanium boride, hafnium boride and tantalum boride. Further carbides which can be used are silicon carbide and boron carbide, and further nitrides which can be used as silicon nitride, boron nitride, aluminium nitride, and thorium nitride. An excellent hard material surface coating is also formed by the mixtures of carbides, nitrides and borides, such as titanium carbonitrides. As for oxides, aluminium oxide and zirconium oxide are preferably used, as well as magnesium oxide, beryllium oxide, thorium oxide, cerium oxide, titanium oxide, hafnium oxide or chromium oxide. The solid solutions of the afore-mentioned oxides, such as chromium oxide and aluminium oxide, as well as mixed oxides of the spinell type, such as magnesium aluminium oxide or magnesium chromium oxide are also used.
In addition to providing a surface coating of hard material on the core of hard metal body, intermediate layers have been provided between the core and surface coating. The main purpose of the intermediate layers is the equalization of stresses. Metals, such as cobalt, nickel and iron have proved particularly suitable for this, also precious metals, such as platinum. The intermediate layers can be applied to the hard metal body by electrodeposition. Intermediate layers can also be formed by the CVD process or one of the PVD processes.
Molded hard metal bodies having a core of a hard metal body and a surface coating of a hard material are known to be very hard at the surface and/or have a low tendency to heatweld. Workpieces made of such surface-coated molded hard metal bodies are therefore very wear-resistant. The surface coating of the hard material generally is formed in such a manner that carbides, nitrides, borides and oxides as well as their mixtures are deposited on the core of hard metal body during a separate process step. For example, deposition from the gaseous phase according to the chemical vapor deposition process is a preferred method of forming a surface coating on a hard metal body.
Tools made of the known hard metal bodies coated on their surface with a hard material have the primary drawback that their use for turning and cutting operations is possible only within limits because the tools are subjected during this use to high impact stresses and strong alternating thermal stresses which often cause the surface coating of hard material to chip off which leads to premature failure of the tools. Hard surface coatings of a layer thickness of more than 20.mu. have particularly poor adhesion to the underlying core of hard metal body. In practice, this means that only hard surface coatings having a layer thickness of between 5 to 10.mu. can be used. Although the wear-resistance of a hard metal body having a surface coating of a hard material should increase with increasing layer thickness of the surface coating, hard surface coatings with a layer thickness of more than 20.mu. cannot be used because under the alternating thermal stresses occurring during use in cutting and turning operations, they come off of their core of hard metal body, before they are worn out, due to lack of adhesion. Attempts have been made to overcome these drawbacks by providing metallic intermediate layers between the core and surface coating or a plurality of hard layers, but these attempts have not been entirely successful.